The present invention generally relates to a system for engine knock control.
Internal combustion engines are designed to maximize power while meeting exhaust emission requirements and minimizing fuel consumption. This objective, however, is often limited by engine knock for a given air-to-fuel ratio. Therefore, it is desirable to be able to detect engine knock such that the engine can operate at its knock limit to provide maximum power and fuel economy. Some engines employ accelerometers to detect engine knock. Because of the low signal-to-noise ratio of conventional accelerometer based knock sensors, a dual-rate count-up and count-down scheme is commonly used for engine knock limit control. These approaches are based upon use of a single knock flag obtained by comparing the knock intensity signal of a knock sensor to a given threshold. The knock intensity signal is defined as the integrated value, over a given knock window, of the absolute value of the signal obtained by filtering the raw knock sensor signal using a band-pass filter. This scheme continually takes the engine in and out of knock, rather than operating the engine at near its knock limit. Moreover, at certain operating conditions, the effectiveness of these schemes may be compromised by engine mechanical noises produced by valve closures and piston slap and picked up by the accelerometers, which may lead to conservative ignition timing and consequent reduced engine performance.
The use of a high quality in-cylinder ionization signal makes it possible to control engine knock using knock intensity derived from ionization signal directly for each cylinder due to the increased signal to noise ratio of ionization versus accelerometer based systems. The cycle-to-cycle variation in the combustion process results in an ionization knock intensity signal that is similar to a random process when the engine is operated at knock conditions. This makes it almost impossible to use a deterministic limit controller to find true knock borderline ignition timing and operate the engine at this corresponding timing. Study shows that the ionization knock intensity feedback signal has great knock controllability. This invention proposes a system and stochastic approach for engine knock control utilizing the mean and standard deviation information of the ionization knock intensity signal as well as the evolution of its stochastic distribution. The proposed stochastic knock limit controller is able to not only seek and find engine knock borderline ignition timing but also operate the engine right at its knock borderline limit continuously.
In view of the above, it is apparent that there exists a need for a system to enable an engine to operate at its knock limit without the occurrence of engine knock or minimal knock.